Feature image: Texas, May 17 2021; Credit To Stormchaser Laura Rowe
Clouds turn the sky into a big art gallery, complete with icy jellyfish and clouds that look like breaking waves. Let’s exlpore!!
These are crisp gaps in mid or high-level cloud layers, below which dangle trails of ice-crystals. To form a fallstreak hole, the cloud layer must consist of supercooled droplets – that is, water is in liquid form – even though temperatures at cloud level are well below 0° celsius. A fallstreak hole forms when one region of the cloud finally starts to freeze, starting a chain reaction. All the moisture from the supercooled droplets in the area rushes to join the ice crystals, which quickly grow big enough to fall below. A form of “virga”, the trail of ice crystals doesn’t tend to reach the ground, but evaporates away before doing so. (Image: Vicki Harrison)
This is a horizontal tube of cloud that forms in the cold downdraughts that spread out ahead of an approaching storm (or, less commonly, at the tail end of a decaying storm). The rapidly sinking air mass can hit the surface so hard that it sends a wave of air gusting some distance away from the storm cloud itself. This cold-flowing air will then slide underneath a layer of warmer air being drawn into the storm’s vertical updraft. As it does so, it condenses the warm air’s vapour into cloud. The resulting roll cloud is completely detached from the main storm cloud. It can often be several kilometres long, as is the case with this impressive example seen heading inland over Shark Bay, Western Australia. (Image: Extraordinary clouds by Richard Hamblyn)
A stack of altocumulus lenticularis clouds hovers over the Alpujarra Mountains in southern Spain, stained red by the rays of the setting sun. Lenticular clouds (Latin for “lens-shaped”) are a common sight in mountainous regions. They have often been mistaken for UFOs. (Image: Extraordinary clouds by Richard Hamblyn)
Lenticular clouds are created when a stable layer of humid air is forced to rise over high ground, condensing its moisture into cloud. If alternate layers of moist and dry air are present, the clouds will form in a vertical stack. Because the airstream returns to its original level once it has passed over the obstacle, it sets up a standing wave effect on the lee side of the moutnain. This image was taken over Mount Rainier, Washington, US. (Image: Ryan Verwest)
When a cap cloud materializes, it tends to look like a hat, perched upon the mountain’s head. It forms when a stable air stream rises to pass over a peak, cooling as it does so. (Image: Michael Davenport)
Amsterdam Island is one of the remotest islands on Earth. It is a tiny volcanic peak, rising from the faultline that separates the Antarctic from the Indo-Australian tectonic plates. Though only 881 metres high, the island can affect the disposition of low cloud over the southern Indian Ocean. In this picture, a layer of humid air, rising and falling as it passes over the island, has formed a sequence of narrow lenticular clouds that rise on the crests of these atmospheric waves. As the cloud-train moves downwind from the island it spreads like a ship’s wake for several hundred kilometres, before blending in with the banks of cumuliform cloud to the north. This image was captured on 19 December 2005 by the Moderate Resolution Imaging Spectroradiometer (MODIS) device on board NASA’s Terra research satellite.
These clouds look a bit like jellyfish floating in the sky. The “tentacles”, which you can see hanging down, are called “virga”. They are essentially rain, or snow, that evaporates away before ever reaching the ground. (Image: Jurgen Oste)
Mammatus clouds (from the Latin for “udders”) are associated with unstable, often stormy weather – though they can also be seen in relatively calm conditions, long after bad weather has passed. Their appearance is the result of pockets of cold, saturated air sinking rapidly from the top of a storm cloud, forming downward bulges or ripples at the base. Their shapes can vary considerably, from long, undulated ripples covering many square kilometres, to smaller patches of near-spherical pouches. This rather menacing display of globular mammatus, contorting itself over a college sports stadium in Hastings, Nebraska in June 2004, is the latter type. Credit: Richard Hamblyn
Looking like enormous waves breaking on the shore, this type of cloud is rare and fleeting – lasting no more than a minute or two. The distinctive breaking-wave shape is caused by wind shear. When a patch of cloud forms at the boundary between a colder layer of air below and a warmer one above, and the upper layer is moving more rapidly than the lower one, the shearing between the two can make undulations appear in the cloud. If the difference in wind speeds is just right, the tops of the undulations are pushed ahead of the bottoms to form dramatic vortices that “break” just like surf on the beach. (Image: Giselle Goloy)